Project Summary Infection with Mycobacterium tuberculosis (Mtb) is a major global health problem in pediatric populations. Children coinfected with HIV and Mtb have an increased risk of developing tuberculosis (TB), even if they are on antiretroviral therapy. We know little about the specific immune defects caused by HIV that are responsible for the increased susceptibility to Mtb, especially in children. As an airborne pathogen, Mtb first encounters immune cells in the lung and the initial response to the infection can dictate whether the host controls the infection or whether the bacteria replicate and spread, causing TB disease. Unconventional T cells, including CD1d- restricted invariant Natural Killer (NK)T cells and Mucosal-Associated Invariant T (MAIT) cells, can detect and destroy Mtb-infected cells and can act before adaptive immunity evolves. HIV impairs both cell types. In our ongoing studies of HIV/Mtb coinfection, using adult macaques and SIV as an HIV surrogate, we found that animals with chronic SIV infection are more susceptible to Mtb and that this is associated with elevated expression of immune exhaustion markers (e.g. PD-1) on MAIT cells. This suggests that SIV-dependent exhaustion of MAIT cells, and perhaps other unconventional T cells, may lower the resistance to Mtb. Here we will use juvenile macaques to model HIV/Mtb coinfected children and determine whether a preexisting SIV infection impairs MAIT and NKT cells. SIV-infected animals will be coinfected with Mtb and TB progression will be quantitatively measured by several clinical, radiologic, and pathologic methods. We will correlate the exhaustion phenotypes of MAIT and NKT cells with the severity of TB, comparing outcomes in SIV-positive vs SIV-nave juvenile macaques. To formally test whether SIV-dependent exhaustion of MAIT and NKT cells impairs TB resistance, we will treat SIV-infected juvenile macaques with anti-PD-1 to reverse immune exhaustion of MAIT and NKT cells. Following Mtb coinfection, we will compare cellular and humoral immune function as well as TB severity in animals treated with anti-PD-1 to those treated with control antibody. We will also leverage an existing pediatric HIV study in Yangon, Myanmar (R01MH108559) to characterize unconventional T cell populations in pre-adolescents with and without HIV infection and HIV/Mtb coinfection. We will use PBMC to determine the relationship between HIV and TB status with peripheral MAIT and NKT cell frequencies, immune exhaustion status, and cellular function. Together, these studies will provide novel insights into the roles of MAIT and NKT cells, as well as immune exhaustion, in HIV/Mtb coinfection of pre-adolescent children and may identify targets for host-directed therapies aimed at improving the health outcomes of children living with HIV.